The present invention relates to a hydraulic control system for controlling clutches or brakes adapted to engage or disengage an element of a planetary gear mechanism in an automatic transmission, and a hydraulic control method in the transmission.
Generally, in a hydraulic control system of an automatic transmission, hydraulic pressure is supplied to hydraulic servos or servomotors adapted to engage or disengage an element of a planetary gear mechanism by selectively changing over a plurality of shift valves. That supply of hydraulic pressure to the hydraulic servomotors is made through a single orifice. The diameter of this orifice is set to such size as to prevent the occurrence of a shift shock by abrupt pressure acting on the hydraulic servomotors, and also to avoid a time lag in speed change.
In the conventional hydraulic control system, however, since the supply of hydraulic pressure to the hydraulic servomotors of the clutches or brakes is made through the single orifice, it may happen that prevention of both the shift shock and the time lag cannot be achieved concurrently, when using one certain clutch or brake for different transmission stages. For example, if the orifice diameter is set so as to prevent the shift shock upon an upshift from the 2nd to the 3rd speed in a direct clutch which is engaged or connected in the forward 3rd or more speed and the reverse condition, a problem occurs in that the hydraulic pressure is restricted excessively at the orifice and hence the time lag in speed change is caused upon a shift to the reverse condition.